This invention relates to printing plates and is concerned with the improvement of plate performance and with a treatment solution for achieving said improvement.
Lithographic printing is a process of printing from surfaces which have been prepared in such a way that certain areas are capable of accepting ink (oleophilic areas), whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.
Plates for use in lithographic printing processes may be prepared using a photographic material that is made imagewise receptive or repellent to ink upon photo-exposure of the photographic material and subsequent chemical treatment. However, this method of preparation, which is based on photographic processing techniques, involves several steps, and therefore requires a considerable amount of time, effort and expense.
Consequently it has, for many years, been a long term aim in the printing industry to form images directly from an electronically composed digital database, ie by a so-called xe2x80x9ccomputer-to-platexe2x80x9d system. The advantages of such a system over the traditional methods of making printing plates are:
(i) the elimination of costly intermediate silver film and processing chemicals;
(ii) a saving of time; and
iii) the ability to automate the system with consequent reduction in labour costs.
The introduction of laser technology provided the first opportunity to form an image directly on a printing plate precursor by directing a laser beam at sequential areas of the plate precursor and modulating the beam so as to vary its intensity. In this way, radiation sensitive plates comprising a high sensitivity photocrosslinkable polymer have been exposed with water-cooled UV argon-ion lasers and electrophotographic plates having sensitivity stretching from the visible spectral region into the near infra-red region have been successfully exposed using low-powered air-cooled argon-ion and semiconductor laser devices.
Imaging systems are also available which involve a sandwich structure which, on exposure to a heat generating infra-red laser beam, undergoes selective (imagewise) delamination and a subsequent transfer of materials. Such so-called peel-apart systems are generally used as replacements for silver halide films.
The present applicants have previously disclosed, in EP-A-514,145 a method of image formation which comprises: providing a radiation sensitive plate comprising a substrate and a coating containing a heat softenable disperse phase, an aqueous soluble or swellable continuous phase and a radiation absorbing substance; Eimagewise exposing the plate to at least partially coalesce the particles of the disperse phase in the image areas; and developing the imagewise exposed plate to remove the coating in the unexposed areas. The directly imaged plates thus obtained may then be used to provide printed images in the normal way using a conventional printing press.
The plates obtained in this way, however, were found to have rather poor durability in printing operations; in particular, they suffered from poor run length on the press. This drawback was believed to be associated with the fact that the at least partial coalescence of the particles of the disperse phase which occurred during imagewise exposure involved a purely physical mixing process. Consequently, it was concluded that more satisfactory performance would be achieved by the use of a system in which new chemical bond formation could be induced in image areas of the plates prior to their use on a printing press, thus providing a greater image toughness and durability.
Accordingly, EP-B-599,510 teaches a method of image formation as previously disclosed in EP-A-514,145, but which additionally comprises the step of heating the developed plate or subjecting it to irradiation to effect insolubilization of the image. In this way, good quality images of high durability are obtained.
Such insolubilization is brought about by chemical reaction between one or more of the components of the coating, which occurs as a result of the heating or irradiation treatment. In order to facilitate such chemical interactions, it is necessary that at least one of the heat softenable disperse phase and the aqueous soluble or swellable continuous phase should include a chemically reactive grouping or precursor therefor.
Despite the improvements which have been effected in this way, however, some further difficulties have been experienced with plates of the type disclosed in EP-B-599,510. In particular, the very short exposure times associated with laser imaging techniques inevitably mean that it is extremely difficult to achieve uniform heating throughout the coating, since the film surface is heated substantially more than those regions well below the surface. As a consequence, surface overheating can occur, causing damage to, or ablation of, the surface material. As well as leading to poor image formation, weak images and potentially impaired press performance, such overheating may also give rise to a plume of ablated debris and pyrolysis products that can attenuate and deflect the imaging laser beam.
Consequently, a system has been disclosed in United Kingdom patent application No. 9709404.9 wherein radiation sensitive plates of this type are provided with an additional, topmost covering layer, said layer having, at the chosen wavelength of exposure, an optical density which is lower than that of the imaging layer. Plates incorporating such a topmost layer achieve more uniform heating through the coating and thereby overcome the difficulties associated with surface overheating; thus, it is possible to obtain improvements in terms of run length, solvent resistance, handleability and scratch resistance.
Surprisingly, however, it has now been found that yet further significant improvements in press life may be achieved by treatment of the imaged plates, prior to post-development baking, with a suitable finishing solution which further enhances coalescence of the particles in the coating.
According to the present invention, there is provided a finishing solution for application to a thermally sensitive printing plate having an imaging layer including particles which are required at least partially to coalesce to form an image, said finishing solution comprising a coalescing aid.
The coalescing aid for use in the finishing solution of the present invention, wherein said plate has a radiation sensitive coating including a disperse phase and a continuous phase, comprises a solvent or mixture of solvents having solubility characteristics which facilitate softening or insolubilization of both said phases, thereby allowing phase separation and reticulation to be avoided. The solubility characteristics may be conveniently expressed in terms of Hansen solubility parameters. Typically, suitable Hansen solubility parameters would fall in the ranges xcex4d (dispersion)=7.0-9.8, xcex4p (polar)=1.5-8.8 and xcex4h (hydrogen bonding)=1.7-5.2 but, for any given solvent, one or more of the parameters may fall outside the specific ranges.
The coalescing aid should also have a boiling point in excess of 250xc2x0 C., preferably in excess of 300xc2x0 C., in order that its total evaporation during the baking of the plate should be avoided.
Preferably, the solvent or solvent mixture which is present in the coalescing aid comprises a ketone, e.g. xcex3-butyrolactone or isophorone, an organic carbonate, for example ethylene carbonate or propylene carbonate, an alcohol such as glycerol or diethylene glycol, a hydrocarbon, e.g. 1,2,3,4-tetrahydronaphthalene (available commercially from E.I. du Pont de Nemours and Company as Tetralin(copyright)), or a dibasic ester of a dicarboxylic acid, most preferably an aliphatic dicarboxylic acid. Suitable aliphatic carboxylic acids are those containing lower alkylxe2x80x94preferably C26 alkylxe2x80x94chains, for example succinic, glutaric and adipic acids. Particularly infavorable results are achieved with the dimethyl, diethyl and dipropyl esters of these acids, and their mixtures. Of most interest in this regard is a mixture of the dimethyl esters of succinic, glutaric and adipic acids, specifically a mixture of dimethyl glutarate (61-67%), dimethyl succinate (20-26%) and dimethyl adipate (13-19%), which is commercially available as DuPont(copyright)DBE or Imasol(copyright)R.
The coalescing aid is advantageously applied to the printing plate in combination with a finishing solution, following exposure and development. Preferably, the coalescing aid is included in the finisher at a level of 0.1 to 5% w/w, most preferably 0.5 to 1% w/w.
The finishing solution typically comprises an aqueous solution containing desensitizers, etching agents and surfactants, and optionally including other additives such as sequestering agents, plasticizers and biocides.
Desensitizers are present in an amount of from 2-10% w/w, preferably from 4 to 7.5% w/w, and serve to prevent sensitization from occurring in background non-image areas, thereby avoiding ink acceptance in these areas, which can otherwise give rise to unsatisfactory prints. Typical desensitizers include sodium gluconate and sodium hexametaphosphate (available commercially as Calgon(copyright)R) and tripotassium citrate.
Cleanliness in background non-image areas, with a consequent avoidance of unwanted ink acceptance and the resulting potential for producing dirty and unsatisfactory prints, is enhanced by the incorporation of an etching agent, such as tartaric acid, in an amount of from 0.2% to 5% w/w, preferably from 0.5% to 2.5% w/w. The etching agent serves to etch the surface of an anodized layer on the substrate, thereby presenting a fresh surface, free from contamination, during printing operations.
Various surfactants, most particularly anionic surfactants, may be incorporated in the compositions and can serve as wetting agents, to enhance the hydrophilicity of non-image areas or, on occasions, as oleophilizers, improving ink acceptance in image areas. Typical anionic surfactants include, for example, sodium diisopropyl-naphthalene sulphonate (available commercially as Rhodacal(copyright)BA77), sodium 2-ethylhexyl sulphonate (available commercially as Surfac EH40) and the sodium salt of naphthalene sulphonic acid-formaldehyde polycondensate (available commercially as Tamol(copyright)7718), and the materials may be present at a level of from 0.1% to 10% w/w, preferably 0.5% to 5% w/w.
Sequestering agents, for example tetrasodium ethylenediaminetetraacetic acid, or glucoheptanoate may be present in an amount of from 0.05% to 2% w/w, preferably from 0.1% to 1% w/w; suitable plasticizers, which may be added at a level of from 0.5% to 10% w/w, preferably from 1% to 5% w/w, include glycerine; and any suitable commercial biocide, such as Bactrachem(copyright)BF2, may be incorporated in an amount of from 0.05% to 2.5% w/w, preferably from 0.1% to 1% w/w. Radiation sensitive plates which may be treated with the finishing solution of the present invention are thermally imaged plates comprising a substrate and an imaging layer, wherein the imaging layer comprises particles which are required to at least partially coalesce to form an image. Said plates are preferably of the type disclosed in EP-B-599510, wherein the imaging layer comprises:
(i) a layer comprising
(a) a disperse phase comprising a water-insoluble heat-softenable component and
(b) a binder or continuous phase comprising a component which is soluble or swellable in aqueous, preferably aqueous alkaline, medium, at least one of the components including a reactive grouping; and
(ii) a substance capable of strongly absorbing radiation to produce heat.
Exposure of such plates to radiation causes at least partial coalescence of the particles in the layer in the exposed areas, thereby forming an image which, due to the presence of the reactive grouping, undergoes insolubilization at elevated temperature and/or exposure to radiation.
Most preferably, plates of the type disclosed in United Kingdom patent application No. 9709404.9 may be treated with the finishing solution of the present invention and produce particularly favorable results. Such plates are essentially of the type previously disclosed in EP-B-599510, but additionally include a topmost covering layer having, at the chosen wavelength of exposure, an optical density which is lower than that of the imaging layer.
The material used for the substrate depends upon the purpose for which the image is to be used and may be, for example, a metal or a plastics material. In the case where the image is to be used as a printing image, the substrate is preferably aluminum, most preferably electrochemically roughened aluminum which includes a surface anodic oxide layer.
The imaging layer may be formed on the substrate using either aqueous or non-aqueous vehicles, or mixtures thereof, in order to obtain a radiation sensitive plate. The imaging layer is preferably coated on to the substrate at a coating weight of 0.1 to 5 g/m2 most preferably 0.8 to 1.2 g/m2 
When it is included, the topmost covering layer may be subsequently coated over the imaging layer using an aqueous, optionally aqueous alkaline, medium to give a layer having a preferred coating weight of 0.01 to 5 g/m2, most preferably 0.1 to 1g/m2 
According to another aspect of the present invention, there is provided a method of forming an image which comprises:
(a) providing a radiation sensitive plate as hereinbefore described;
(b) imagewise exposing the radiation sensitive plate to a beam of high intensity radiation by directing the radiation at sequential areas of the coating and modulating the radiation so that the particles in the imaging layer are selectively at least partially coalesced;
(c) developing the imagewise exposed plate with aqueous medium to selectively remove the areas containing the non-coalesced particles and leave an image on the substrate resulting from the at least partially coalesced particles;
(d) treating the developed plate with a finishing solution according to the present invention; and
(e) heating the finished plate and/or subjecting it to actinic radiation to effect insolubilization.
In a particular embodiment of the invention, the source of the high intensity radiation is a laser operating in the ultra-violet, visible or infra-red region of the spectrum. Red and infra-red light emitting lasers are typically used, for example the semiconductor or diode lasers, typical of which is the gallium aluminum arsenide laser which operates in the 750-870 nm region, and neodymiumxe2x80x94YAG lasers which operate around 1064 nm.
Preferred developers for selectively removing the non-coalesced material in the non-image areas are aqueous alkalis, such as solutions of ethanolamine and sodium metasilicate, an alkaline phosphate such as trisodium phosphate, or an alkali metal hydroxide in water.
Plates treated prior to baking with the finishing solution of the present invention show improved press performance, in terms of run length and image definition, and are also characterized by greater solvent resistance, increased durability of highlights on press and increased crosslink density following the baking step. In addition, image formation requires a lower energy of exposure than in the case of plates treated with the finishing solutions of the prior art, and the conditions required during the post-finishing baking treatment are less stringent, in terms of both temperature and time, resulting, in each case, in significant cost savings.
The following examples are, without limitation, illustrative of the invention.